The present invention relates generally to an apparatus and method for flicker filtering. More particularly, the invention relates to an apparatus and method for significantly reducing flicker in a video display by employing a two-dimensional flicker filter.
A video image, such as that found on a television for example, is formed by a succession of flanes projected onto a phosphorescent screen, such as that found in a cathode-ray tube (xe2x80x9cCRTxe2x80x9d). Multiple horizontal lines of pixels with many pixels per line, in turn, form each fame.
To draw each frame, an electron beam in a CRT scans horizontally along each horizontal line. As it projects each pixel of a horizontal line in turn, the beam supplies energy to the phosphors which phosphoress, thus illuminating the pixel. The amount of energy supplied by the electron beam sets the initial intensity of the pixel, but the intensity pixel progressively degrades between scans as its phosphors give up energy in the form of light. Each pixel therefore acts like a small light that flickers at the frequency at which the pixel is scanned.
However, if the frequency at which a pixel is scanned is high enough, at least 45 Hz, a viewer will not perceive pixel flicker. This is due to the fact that past this threshold, humans can not cognitively process the optical changes fast enough in order to perceive the flicker. Typical television systems typically, however, do not update pixels fast enough. For example an National Television Standards Committee (xe2x80x9cNTSCxe2x80x9d) has established television and video standards in the United States (in Europe and the rest of the world, the dominant television standards are PAL and SECAM) which defines television standard as a composite video signal with a refresh rate of 60 half-frames (interlaced) per second. A television operating on the NTSC standard then updates each pixel 30 times per second, or 30 Hz. Thus, a viewer can readily perceive pixel flicker in an NTSC format display. Phase Alternating Line (xe2x80x9cPALxe2x80x9d), the dominant television standard in Europe has a similar problem.
The NTSC and PAL formats use interlacing to help mask pixel flicker. Each frame is divided into two interlaced fields. One field includes all even numbered pixel rows while the other field includes all odd numbered pixel rows. When displaying a frame, all the rows of one field are scanned and then all the rows of the other field are scanned. Thus, two vertically adjacent pixels will flicker almost 180 degrees out of sync. Since the two pixels are adjacent, they will usually have the same or nearly the same intensity, particularly when the image does not have sharp horizontal edges. In an NTSC system, the two pixels will look like a single pixel flashing at 60 Hz instead of two pixels flashing at 30 Hz each. Since 60 Hz is above the 45 Hz threshold level for flicker perception, the viewer will not perceive that the two pixels flicker.
Thus, vertically adjacent pixels tend to compensate for each other""s flicker.
However, an image with sharp contrasts at its edges, such as a bright rectangle displayed on a dark background for example, can be problematic.
The upper or lower edge of the rectangle acts as a sharply defined horizontal boundary between areas of high and low intensity. For example, a row of pixels immediately below the horizontal lower edge of the rectangle flickers with high intensity while the row of pixels immediately above the lower rectangle edge flickers with little or no intensity. Thus, the flicker of the low intensity row of pixels will not adequately compensate for flicker of its high intensity neighboring row, and a viewer will perceive flicker in the high intensity row.
One way to reduce flicker along a horizontal intensity boundary is to filter the signal controlling the beam so as to reduce the abruptness with which image intensity changes in the vertical direction. A prior art xe2x80x9cone-dimensionalxe2x80x9dflicker filter sets the intensity of each pixel to a weighted average of itself and its nearest two vertical neighbors. The intensity of each pixel therefore increases when a vertical neighbor is brighter and decreases when a vertical neighbor is dimmer. This reduces flicker because it ensures that vertically adjacent neighbors will flash with more nearly the same intensity. Such filtering blurs sharp horizontal intensity boundaries making an image appear fuzzy, but most viewers are willing to give up sharpness at horizontal intensity boundaries in order to reduce annoying flicker. However, such a flicker filter is not selective and thus, adjusts pixel intensities everywhere in the image, not just in areas of the image where flicker is a problem. This has the effect of reducing image sharpness thereby reducing image quality.
Accordingly, it is an object of this invention to provide a flicker filter that reduces flicker in an image.
It is another object of this invention to provide a flicker filter that maximizes image quality in two dimensions.
It is still another object of the invention to provide a flicker filter that allows a user to selectively adjust input values to compensate for variable boundary conditions.
These and other objects of the invention will be obvious and will appear hereinafter.
The aforementioned and other objects are achieved by the invention which provides a flicker filter for reducing flicker in a video signal. Often, such a filter is located within a television or a video scan converter.
The flicker filter of the invention has at least two user-adjustable inputs adapted to balance image quality versus flicker in the second video image. A first user-adjustable input is adapted to govern an amount of flicker suppression. A second user-adjustable input is adapted to govern an amount of blur, or sharpness.
The two inputs are independently adjustable such that a user may dynamically adjust the two-dimensional system characteristics. This ability allows the circuit to be time to adjust pixel intensities where an amount of pixel intensity adjustment increases with decreasing boundary angle, inter alia.
In further aspects, the invention provides methods in accord with the apparatus described above. The aforementioned and other aspects of the invention are evident in the drawings and in the description that follows.